Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($)	6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserves discount	$ 28,000,000	$ 26,000,000
Increase (decrease) in estimates for claims	17,000,000	(7,000,000)
Adjustment expense	1,000,000	2,000,000
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable reserve development net of premium offsets	19,000,000	5,000,000
Reinsurance and Monoline Excess Segment
Liability for Claims and Claims Adjustment Expense [Line Items]
Unfavorable reserve development net of premium offsets		$ 7,000,000
Favorable reserve development net of premium offsets	$ 20,000,000